Industrial fermentation predominantly uses glucose as a feedstock for the production of a multitude of end products such as enzymes, proteins, amino acids, organic acids, sugar alcohols, pharmaceuticals and other biochemicals. In many applications glucose is produced from the enzymatic conversion of substrates comprising starch and cellulose (e.g. whole milled cereal grains). Starch, which comprises two polysaccharide fractions, amylose and amylopectin, is deposited in plant cells as granular particles. The partial crystalline structure of these granules imparts insolubility in cold water, and, as a result, solubilization of starch granules in water generally requires heat energy to disrupt the crystalline structure of the granule. Numerous processes have been employed for starch solubilization and these include direct and indirect heating of substrates comprising granular starch. (See, for example, STARCH CHEMISTRY AND TECHNOLOGY, Eds R. L. Whistler et al., 2nd Ed., 1984 Academic Press Inc., Orlando Fla. and STARCH CONVERSION TECHNOLOGY, Eds G. M. A. Van Beynum et al., Food Science and Technology Series 1985 Marcel Dekker Inc. NY).
Starch to glucose processing generally consists of two steps and these steps include liquefaction of starch and saccharification of the liquefied starch. Further steps can include (a) purification and isomerization when the desired end product is a purified dextrose or fructose or (b) fermentation and distillation when the desired end product is, for example an alcohol (e.g., ethanol).
An object of the starch liquefaction process is to convert a slurry of starch polymer granules into a solution of shorter chain length dextrins of low viscosity. This is an important step for convenient handling of industrial equipment used in starch conversion processes. Commonly, the starch is liquefied by use of high temperature and enzymatic bioconversion. For example, a common enzymatic liquefaction process involves adding a thermostable bacterial alpha amylase (e.g. SPEZYME® PRIME and SPEZYME® FRED, SPEZYME® ETHYL (Danisco U.S., Inc, Genencor Division) or TERMAMYL SC, TERMAMYL SUPRA or TERMANYL 120L (Novozymes)) to a slurry comprising a substrate including starch and adjusting the pH to between 5.5 to 6.5 and the temperature to greater than 90° C. The starch is liquified and then subjected to saccharifying enzymes. Typically, saccharification takes place in the presence of glucoamylase enzymes such as glucoamylase from Aspergillus niger (e.g., OPTIDEX L-400 (Genencor International Inc.)) at a pH more acidic than the pH of the liquefaction step.
A number of variations exist for the liquefaction and saccharification of a starch substrate. However, there is a need for more efficient means for starch liquefaction, saccharification and fermentation.